Laser marking is commonly used on bearing components, to provide the component with e.g. a type number. Laser marking for bearing steels is based on the changes that develop in the material due to the heating of the steel by the laser radiation. Various ways of laser marking may be distinguished. Some of them are described hereafter. Engraving is the removal of material to a depth of 10 to 50 μm. The laser is used to create a groove in the material and due to melting and evaporation, material may be removed to form the groove. Ablation is the evaporation of material by extremely fast laser pulses. The processed material evaporates and fumes are exhausted to prevent re-deposition. For metallic pieces ablation may be used, for example, for surface patterning or for drilling extremely fine holes. Tempering, also called annealing, is the main method for marking steel components. The heat of the laser tempers the steel which changes reflectivity and/or surface structure, leading to a visible mark.
WO2007/020257 discloses a method of marking a metallic object by local conversion of its metallic structure, for example by heating with a laser. This document describes that the conversion takes place through the thickness of the piece to be marked.
It is also known to create marks through the modification of the structure or internal stress within materials, for example by laser treatment. According to U.S. Pat. No. 6,527,193, this creates local deviations in the magnetic, acoustic or electric properties of the marked material, these deviations being hidden to the eye but readable with appropriate instruments.